Proper regulation of photoreceptor-specific genes is essential for the maintenance of retinal function and integrity. Despite the availability of considerable information on the regulation of opsins and other specialized genes associated with the terminal phases of photoreceptor differentiation, relatively little is known about the regulation of a select group of genes directed for expression in both rods and cones at a time early in photoreceptor differentiation. The gene encoding rhodopsin kinase (Rk) is one such early gene which plays a crucial role in the adaptation and recovery of rod and cone photoreceptors and protects these cells against light-induced damage. Mutations in Rk like those in other photoreceptor-specific genes cause human photoreceptor dysfunction, retinal disease and visual compromise. Rk, however, differs from other genes of terminally differentiated photoreceptors in that its expression appears unaffected by targeted disruption of the major known cone-rod homeodomain transcription factor, Crx. Investigation into the mechanisms that control the transcription of Rk is therefore likely to provide a view of the novel Crx-independent molecular network responsible for regulation of this and other early genes and the early phases of photoreceptor differentiation. The preliminary work in this lab on Rk regulation and promoter function in transgenic mouse lines, cell cultures and biochemical assays already support the role of a separate Crx-independent transcriptional mechanism. The proposed study will focus on further characterization of this regulatory system in relation to Rk. The first aim will be to define spatial and temporal patterns of Rk expression and evaluate the requirements for promoter function in Rk-GFP transgenic mice. The second aim will focus on delineation of the regulatory sequences responsible for Rk promoter activity and specificity in primary retinal cultures and retinal precursor cells. Finally, in the third aim the interaction of known and unidentified transcription factors with various key elements within the promoter including a functional homeodomain core and a GA-box sequence will be explored. These studies will serve as a basis for future work leading to the identification and characterization of novel trans-acting factors which may play crucial roles in photoreceptor differentiation and cell fate determination. These novel candidate genes may also participate in the pathogenesis of human retinal degenerations.